1. Field of the Invention
The present invention relates to a hydraulic control circuit for a hydraulically operated vehicular transmission having at least five forward transmission trains.
2. Description of Related Art
A hydraulically operated vehicular transmission is so arranged that, by using a plurality of hydraulic engaging elements (clutches and brakes), the supply of hydraulic oil to, and discharge thereof from, these hydraulic engaging elements are controlled by a plurality of shift valves so as to establish each of the transmission trains. In this case, there are provided shift valves that are smaller in number by one than the number of transmission trains. For example, in the case of five transmission trains, four shift valves are provided, i.e., 1-2 shift valve, 2-3 shift valve, 3-4 shift valve and 4-5 shift valve. These shift valves are respectively made to be capable of being changed over between the two positions of a low-speed side and a high-speed side. At the low-speed position of each shift valve, a hydraulic engaging element for the transmission train to be taken care of by each of the shift valves is connected to an oil passage which is communicated with an oil passage on an upstream side of the shift valve. At the high-speed position of each shift valve, the oil passage on the upstream side thereof is connected to an oil passage that is communicated with a shift valve on the downstream side. In this arrangement, when the 1-2 shift valve is at the low-speed position, the first-speed transmission train is established, when the 1-2 shift valve is at the high-speed position and the 2-3 shift valve is at the low-speed position, the second-speed transmission train is established, when the 1-2 shift valve and the 2-3 shift valve are at the high-speed position and the 3-4 shift valve is at the low-speed position, the third-speed transmission train is established, when the 1-2 shift valve, the 2-3 shift valve and the 3-4 shift valve are at the high-speed position and the 4-5 shift valve is at the low-speed position, the fourth-speed transmission train is established, and when all of the shift valves are at the high-speed position, the fifth-speed transmission train is established.
In order to effect an automatic transmission or speed-changing by using an electronic control circuit such as a microcomputer or the like, there is conventionally known a transmission in which the shift valves are controlled for changeover by solenoid valves to be opened or closed by the electronic control circuit. In this case, normally one solenoid valve is provided for each shift valve. However, in order to reduce the number of solenoid valves, there is also known a transmission in which four shift valves in a five-stage transmission are controlled for changeover by three solenoid valves (Japanese Published Unexamined Patent Application No. 88353/1988).
If the number of transmission trains is increased, aside from the increase in the number of hydraulic engaging elements, the number of valves is also increased, with the result that a valve block to be contained in a transmission case becomes large. Consequently, the transmission itself becomes large and its weight also increases.
As described above, if four shift valves are provided for the five-stage transmission, the increase in size of the valve block cannot be avoided even if the solenoid valves are reduced to three in number.